Proceedings of the 20th Lwwr and Planetary Science Co nference, pp. 54 1-55 54 1 Lunar and Planetary Institute, Houston, 1990
Geologic Setting of Diverse Volcanic Materials in Nlrthern Elysium Planitia, Mars
P.J. Mouginis-Mark Plnnetary Geosciences Division, Hawaii Institute of Geophysics, Universi of Hawaii, Honolulu, HI 96822
Geologic mapping of high-resolution (30-50 m/ pixel) VIking Orbiter images Jnorth ern Elysium Planitia has identified seven sites where current problems in martian volcanolo'Y, chronology, and stratigraphy can be resolved. These sites, which are discussed in the context of a wtential Mars rover/ sample return mission, would permit the foU owing investigations: ( 1 ) the dating o Lower Amazonian lava flows from Elysium Mons (thereby providing absolute calibration for global crater size/ frequency relative chronologies, ( 2) the petrologic investigation of long run-out lava flows, ( 3) the geologic interpretation of materials that may either be lava flows or lahar deposits, ( 4) the balysis of materials believed to be ash deposits produced by explosive eruptions of Hecates Tholus, and f5) the investigation of the stratigraphy of fractured terrain along the boundary between northern Elysium t;'lanitia and southern Utopia Planitia. I
INTRODUCfiON frequency curves derived from orbital data can be absolutely calibr.tted, (2) to permit petrologic studies of Elysium Mons For more than a decade, analyses of the Viking Orbiter image lava flows, ( 3) the geologic!interpretation of materials that may data set have revealed the diversity of volcanic materials on either be lava flows or lahar deposits, ( 4) to study the physical Mars from scales ranging from < 1 km to >1 OOOs km. Global properties (the degree of cementation and particle size mapping at 1:25M (Scott and Carr, 1978) and 1:15M (Scott distribution of the surfaoe, stratigraphic relationships, and and Tanaka, 1986; Greeley and Guest, 1987), as weU as microtopography) of the volcano Hecates Tholus in order to numerous regional geological studies at 1:2M, have motivated better define its eruptive history, and ( 5) to study stratigr.tphic detailed mapping at 1:SOOK under NASA's Mars Geologic sections of the boundary arp that separates Elysium Mons Mappers Program in order to resolve local geologic relation lava flows from the northern plains. ships. One of several areas identified as important to the interpretation of the volcanic history of Mars is northern ASSUMED ROVER CAPABILITIES Elysium Planitia. Materials resulting from both lava-producing and explosive volcanic eruptions are believed to exist here For the purpose of this · vestigation, it is assumed that the (Mouginis-Mark et al., 1982, 1984; K. L. Tanaka, M.G. rover will land at the same locality as the sample renu-n vehicle Chapman, and D. H. Scott, unpublished data, 1989), as weU as and that selected samples can be renu-ned to Earth in addition the products of complex interactions between volcanic to the rover performing in situ investigations. Alternative materials and subsurface volatiles (Mouginis-Mark, 1985). To configurations have also been di.scussed (Blanchard et al., the west and north of the volcano Hecates Tholus, numerous 1985; Mars Study Team, 1987; Gooding et al., 1989), but lava flows from Elysium Mons, coUapse features, plains units, these configurations would not significantly affect the science and possible melt water deposits have been identified by these objectives as they are deS
Fig. 1. (a) Overview of the pro posed rover traverse site in north ern Elysium Planitia. Proposed landing site is marked by a star, and the six science stations are num bered I through 6. Portion of U.S. Geological Survey photomosaic MTM-35212. Width of image equi valent to 190 km, north is to the top. (b) Geologic map of the pro posed rover traverse site in north em Elysium Planitia (area shown is the same as that in a). Unit abbre B viations are as follows: "HT" - Hecates Tholus, "Fu" - undi.fferen
tiad flows, "F1" - lobate fl ows, "Np" - Northern Plains materials, "UfB" - upper fractured boundary materials, "liB" - lower fracn1red boundary materials, and "Ce" - crater ejecta. Rim crests of meteor ite crdters are marked by barbed circles. Mouginis·M rk: Volcanic materials in Ely sium 543 candidate landing sites will have to come under greater Earth year life expectan should be planned for the rover. scrutiny for more detailed site selection dwjng the Mars Assuming 10 hours of tra erse time per martian day, and two Observer (MO) Mission, when such sites could be imaged at days spent at each sci I ce station and the landing site a resolution of - 1.5 m with the MO high-resolution camera. (including three visits to return samples), the rover would 2. A key aspect of the mission would be the return to Earth have to travel at a mean SReed of - 50 m/ hour when traversing of at least three selected samples that are obtained from the martian surface. I documented in situ localities. The rover must therefore be capable of visiting suitable localities identified either from orbit THE riNG SITE or from terminal-descent imaging. It is assumed here that each The landing site choseffor this study is located at 33.0° N, of these sample localities may be up to 10 km from the return 212.4°W and is devoid of eteorite craters larger than - 300m of the sapcecraft. An extended surface traverse will begin once in diameter (Fig. 1). Alth ugh no good estimates of absolute the collected samples are placed within the sample return elevation exist for this site, the elevation is believed to be about vehicle. 5 km (US. Geological Survey Map l-1120, 1978). However, 3. The rover vehicle must have a range capability of stereogrammetrically derived data for the area surrounding - 200 km over surfaces expected to locally J?Ossess a few Elysium Mons (Blasius nd Cutts, 1981) show that the meters of vertical relief (i.e., comparable to the topography on elevation of the proposed landing site may be less than 3 km terrestrial aa and pahoehoe flows; B. Campbell, S. H. Zisk, and above the 6.1 mb mean Mars datum (Earth-based radar P. Mouginis-Mark, unpublished data, 1989) and have boulder topography data of Downs et al. ( 1982) can be used to populations comparable to those seen at the Vtking Lander provide an absolute reference datum for the stereographically sites ( Garoin et al., 1981 ). The rover will also have the ability derived elevation]. to climb (or descend) slopes of - 2° -3° for traverses perhaps In addition to represen ing a safe landing site, the primary 10-15 km in horizontal extent and to communicate directly criterion for selecting t~ landing site is the collection and with Earth (i.e., there will be no need for the rover to return to Earth of a sample of the plains materials that underlie communicate with Earth via the landing craft). An alternative the lobate lava flows from northern Elysium Mons. The plains way to achieve this goal is for the rover to communicate with materials are interpreted to be undifferentiated lava flows of Earth via a Mars-orbiting relay station (Mars Study Team, Lower Amazonian age that form part of the Elysium formation 1987). (Unit Ael of Greeley and Guest, 1987) and have a crater 4. The instrument complement of the rover will depend 1 frequency of - 250 cratJ rs larger than 2-km diameter per upon the specific mission science requirements, but, as 6 2 10 km . Because these Unit Ael flows cover a large portion discussed by Singer ( 1988), it should carry suitable instru 1 of Elysium Planitia, obtaining a sample would enable an ments that will enable it both to navigate across the martian absolute age-calibration point to be added to the crater surface and to perform sample identification. frequency curves that ar currently used to interpret relative For the proposed northern Elysium traverse the minimum martian ages (Neukum and Hiller, 1981; Albee, 1988). capabilities and payload of the rover would comprise: ( 1 ) an Neukum ( 1988) has argued that it is particularly important onboard computer that has sufficient artificial intelligence to to obtain an absolute age for Lower Amazonian to Lower permit terrain navigation without frequent delays due to the Hesperian materials, beduse these materials evidently were round-trip time involved with communications with Earth erupted during a transiti?n from a decaying crater flux to a (Solar System Exploration Committee, 1986, pp. 226-227); steady-state crater flux. Determining the radiometric age for ( 2) a video camera system (probably stereographic) for terrain Lower Amazonian sample~ may thus aid in the determination navigation; ( 3) an imaging spectrometer that has moderate- to of the absolute age of the inflection point in the martian high-spectral resolution for mineral characterization and cratering rate, thereb defining much of the absolute sample selection with a spatial resolution sufficient to identify chronology for the planet. individual rock samples and, preferably, mineral grains at close proximity; ( 4) a drill capable of obtaining unweathered samples from rock outcrops; ( 5) a surface sample that will be GEOWGIC OBJECfiVES able to conduct trenching experiments comparable to those In order to define the range of volcanic materials in northern by the Vtking Landers (Moore et at., 1982) to enable the Elysium, six science stations have been investigated via analysis investigation of materials at depths of 10-20 em beneath the of the 30-50 m/ pixel Vtking Orbiter images of this area. The surface; ( 6) a gravimeter for measuring local gravity anomalies; geologic objectives at eac? station are as follows. and (7) an active seismic experiment for the investigation of shallow crustal structure (within - 5 km of the surface) along Station 1 the rover traverse. The life expectancy of the rover is not defined as part of Seven kilometers to the east of the landing site is a this analysis, but it is presumed that the vehicle will have the prominent lobate Java flo that appears to have been erupted ability to operate for a sufficient duration on the martian from the northern flanks of Elysium Mons (Fig. 2). This flow surface to complete the traverse. The traverse described here can be traced for - 150 km toward the summit of Elysium is - 165 km in length, and science investigations at each station Mons before Viking Orbiier image resolution is insufficient to would involve drilling, sampling, and imaging, so that a one resolve individual flow l bes. This lava flow is one of many 544 Proceedings of the 20th Lunar Planetary Science Conf erence
Fig. 2. View of the proposed site (star) and Science Stations 1 and 2. The alternate landing site, which would reduce the total traverse distance by - 35 km ( does not include Station 2) is marked by "+". Station 1 (32.70°N, 211.35°W) is located at the edge of a lobate lava flow that erupted from the northern flank of Elysium Mons. Station 2 (32.72° N, 211.64°W) is at the edge of an enigmatic flow that could the product of volcano/ground-ice interactions (Mouginis-Mark, 1985). Mosaic of Viking Orbiter images 651Al0-15, which have a of 52 m/pixel. Image width is equivalent to 65 km and north is to the top.
such flows associated with Elysium (and other martian Station 2 volcanoes) that are very long 1 to terrestrial examples This station is located 8 krn to the west of the landing site (e.g., Wood, 1984; Cattermole, 1 ). The reason these flows where there is an unusual low-relief flow that does not appear are so long is poorly known. By with lava flows on to be a lava flow based on its subdued morphology (Fig. 2). Earth, martian flows may attain such Such flows not only lack lobate edges and the rugged surface rate (Walker, 1973 ), mafic L"'-"u'"+, texture of lava flows on Mars (1beilig and Greeley, 1986) but viscosity ( Cattermole, 1987), or are also evidently thinner than the lava flows in this area. their propensity to form tube-fed Mouginis-Mark (1985) proposed that these unusual flows are Additionally, super-elevated temperatures similar to similar to Icelandic jokuJhalups, produced by the release of those inferred for terrestrial (Basaltic Volcanism sediment-laden melt water by the interaction of magma and Study Project, 1981 ) may lead high effusion rates. A layers of ground ice. returned sample from this lava thus not only would provide an age date for the eruptions of Elysium There is increasing morphologic evidence that interactions Mons, but also would permit the g~ochemistry of such long between magma and subsurface volatiles took place over an runout lava flows to be investigated. It is expected that the extended period of martian geologic history (Allen, 1979; collected sample from Station 1 w uld be returned to the Mouginis-Mark, 1985; Squyres et al., 1987). Mouginis-Mark ascent vehicle prior to visiting Statiop 2, in order to minimize ( 1985) hypothesized that within northern Elysium Planitia this mission loss should the rover fail durihg the visit to Station 2. release of melt water may have been related to late-stage Volcanic materials in Ely sium 545
Fig. 3. Station 3 (located at 33.37°N, 21l.12°W) is at the boundary between Hecates Tholus right) and the undifferentiated lava flows from Elysium Mons. lbis station is key to identifying the relative age of the systems that are found on Hecates Tholus; channel deposits on the plains would indicate a relatively recent period of erosion after the formation of the volcano and would considerably extend the time over which the volcano may have been active. Statio~ 4 (located at 33.30°N, 210.93°W) is located on the flanks of Hecates Tholus, at a location where the lack of major channels would pennit the rover to ascend and descend the flanks. Either drill core or trenching experiments could be performed at this location in orde~ to investigate the possible explosively derived origin of these volcanic materials. Viking Orbiter frame 86A38, resolution 38 m/pixel. Image width is equivalent to 40 km and north is to the top. 546 Proceedings of the 20th Lunar and Planetary Science Conference
intrusive events rather than the emplacei:Dent of lava flows at between these types of deposits will depend upon analyses of the swface (i.e., comparable to events hypothesized to occur the grain size distribution, thickness, and detailed stratigraphy in other areas of Mars by Squyres\ et al., 1987), and so the within the deposits. Comparable facies studies have been age of the jokulhalup deposits may be considerably younger conducted on Monte Vulture Volcano, Italy, to discriminate than the adjacent lava flows. The goals for collecting and between the ignimbrites and lahar deposits (Guest et al., returning to Earth a sample from ~ite 2 are therefore: ( 1 ) to 1988); cross-bedded surge deposits were found to be identify the process by which the flow was emplaced, associated with variable thicknesses of ignimbrites (analogs to specifically distinguishing (via visual observations made from the volcanic debris avalanches desc.ribed by Reimers and the rover's video camera) between a volcanic origin and a Komar ( 1979 ), while lahars (water-laden flows) tended to fill chaotic debris flow produced by volcano/ground-ice interac preexisting depressions and have relatively uniform thickness tions; and ( 2) to obtain a relative kge determination for this in longitudinal section. In addition, terrestrial lahars (and material (via an analysis of the degree of development of probably martian examples, too) may be distinguished from weathering rinds on materials within the jokulhalup deposit other volcaniclastic and fluvial deposits by a greater abundance and the adjacent lava flows) to see how recently the of day-sized particles and the presence of extremely large hypothesized subsurface volatiles may have been released to boulders (Fisher and Schmincke, 1984, pp. 307-311). Water the swface. Should the material at Station 2 indeed prove to carved channels are expected to have fine-grained lacustrine be related to melt-water release, obtaining an age estimate for deposits at the margins of the flows. In the case of the Elysium the days and/ or other fine particles expected to be within the examples, morphologic evidence for late (post-Elysium Mons jokulhalup deposit would help coostrain the poleward lava flows) channel formation on the flanks of the volcano migration of subswface volatiles and the probable depth of ice would be particularly important to search for with the rover. as a function of geologic time (as modeled by Fanale et al., If such evidence were found, this would imply that volatile 1986). The sample from Station 2 would also be renrrned to release took place from Hecates Tholus in the relatively recent the ascent vehicle prior to the rover starting the traverse to history of Mars (i.e., postdating the age of the lava flows Station 3. It is assumed that after the collection of samples from sampled at the landing site). the landing site, Station 1, and Station 2 the renrrn stage of the lander would begin its renrrn to Earth while the rover initiates Station 4 a more far-ranging exploration of the region. Hecates Tholus has a basal diameter of - 190 km, and is believed to rise - 6 km above the surrounding plain (Hard et Station 3 al., 1974). e science objective at Station 4 is primarily to 1\vo scientific sites are located dn the northwest flank of investigate the origin of the materials on the flanks of the
the volcano Hecates Tholus (Fig] 3 ) 1 which lies - 80 km highland patefe. In order to access Station 4, the rover must northeast of the landing site. Of par\:icularly scientific interest be able to ascend and descent slopes of 3-4 degrees for a at Station 3 would be the analysis of materials that are expected distance of - 5 km (Fig. 3 ). Although the physical strength of to lie at the distal ends of numerous digitate channels that are the flanks is unknown, it is likely that the volcano's surface very numerous on all flanks of the volcano. Although the lower materials are less consolidated than the adjacent Elysium Mons slopes of the Hecates Tholus edific are embayed by Elysium lava flows, as the flanks appear to be relatively easy to erode Mons lava flows, these channels are of uncertain age and origin and have permittted the formation of numerous digitate and thus could have formed after mplacement of the lavas. channels. Thus, although vehicle traction and surface gullying Channels of this type are quite co on features on the older on these slopes may reduce the ability of the rover to reach highland patera of Mars, such as Hadriaca, Tyrrhena, and Station 4, it i likely that the flanks of Hecates Tholus are Apollinaris Paterae (Reimers and K9mar, 1979), and the two probably sufficiently consolidated to support a rover as no most likely modes of formation are that they were carved by evidence of wind erosion of friable materials (e.g., yardangs) volcanic debris flows (Reimers and {omar, 1979) or that they can be found on the flanks. were water-carved by volatiles re ~ease d from the volcano Hecates Tholus is comparable to Hadriaca, Tyrrhena, and (Mouginis-Mark et al., 1982). The implications of either Apollinaris Paterae in that each of these volcanoes have mechanism for martian volcanology\ are of basic importance channelized flanks but lack any lobate lava flows (Reimers and not only for identifying the range o eruption styles ( Wllson Komar, 1979). It has been proposed by Mouginis-Mark et al. et al., 1982 ), but also for investigatiljlg the role that volcanoes ( 1982) that I-tecates Tholus experienced numerous explosive may have played in providing inputs to the early martian (plinian) eruptions that produced flanks comprised of ash atmosphere (Postawko and Kuh 1986; Greeley, 1987b; deposits rather than lava flows. Similar models for the early Wilson and Mouginis-Mark, 1987). histories of the volcanoes Tyrrhena Patera (Greeley and Spudis, It is expected that in situ morphologic investigations of the 1981) and Alba Patera (Mouginis-Mark et al., 1988) have also distal ends of these channels will resdlve not only whether they been proposed. Rover investigations of the physical properties were carved by flowing water, wat -laden debris avalanches of the flank deposits of Hecates Tholus, particularly studies that (lahars), or volcanic debris avalanches, but also whether they involve the use of a trenching tool to dig a few tens of pre- or postdate the emplacement\ of the lava flows from centimeters beneath the surface of a (possible) duricrust layer, Elysium Mons. Recognizing the rorphologic differences would help resolve a crucial issue pertaining to the diversity Mouginis-Mark: Volcanic materials in Elysium 547
Fig. 4. Station 5 (33.84°N, 210.88°W) is selected to provide sampling opportunities for the analysis of the isolated massifs that charJ.cterize the northern plains in Utopia Planitia. Currently unresolved is the issue of whether these massifs are remnants of a once more regionally extensive cap-rock of Elysium Mons lava flows or, alternatively, basement materials that have now been almost completely buried by sediments within the northern plains. VIking Orbiter image 86A39, resolution 38m/pixeL Image width is equivalent to 40 km and north is to the top. 548 Proceedings of the 20th Lunar and Planetary Science Conference
of styles of volcanic eruptions on Mars. If the flanks of Hecates The primary science objective at Station 5 is to investigate Tholus were indeed produced by plinian eruptions as the morphology of the surficial deposits at this site and, via proposed by Mouginis-Mark et ai. (1982), then layers of ash the imaging spectrometer, attempt a spectroscopic determina deposits should be found, thereby indicating that the early tion of the composition of the isolated massifs that project volcanic history of Mars was significantly different from the through the surficial cover of the plains materials. On activity that characterized the late-stage (Amazonian) lava stratigraphic grounds these massifs may be some of the oldest producing eruptions on the Tharsis shield volcanoes (Carr et materials exposed in this region of Mars, or, alternatively, they ai., 1977a; Scott and Tanaka, 1986). Alternative interpreta may be remnants of the Elysium Mons lava flows that have been tions for the Hecates Tholus flank deposits include pyroclastic down-dropped in the rest of this area and almost completely flows [as hypothesized for Alba Patera (Mouginis-Mark et ai., buried by the younger, presumably sedimentary, deposits. 1988), which would also imply explosive volcanic eruptions] or, possibly, hydrothermally altered lava flows. Station 6 A separate issue relating to the possible ash deposits on the Exploration of an area of chaotic terrain commences at flanks of Hecates Tholus is the determination of the magma Station 6, where the rover can gain easy entrance to a series chemistry. Francis and Wood ( 1982) have argued against of interconnected canyons (Fig. 5). These canyons measure silicic explosive volcanism on Mars, but it is theoretically about 10-15 km in length, have widths of - 500 m-1 km, and possible for Mars to have experienced basaltic plinian are estimated from shadow length measurements to be about eruptions (Wilson et ai., 1982). Identifying silicic materials on 400-600 m deep. This area was described by Carr and Schaber Mars would thus have significant implications for crustal ( 1977) as a likely area for the collapse of surface materials differentiation and would also be relevant to the current following the release of subsurface volatiles, and further debate about the mode of formation of the enigmatic deposits mapping of the boundary scarp by Mouginis-Mark (1985) within the Amazonis region of Mars, which Scott and Tanaka supports the idea of melt-water release from the scarp (1982) hypothesize are ignimbrite deposits. Although surficial following the emplacement of lava flows from Elysium Mons. deposits may be interpretable from orbit, by using the imaging At Station 6, depending upon the degree of talus covering the spectrometer onboard the rover it would be possible to slope f.tce, the stratigraphy of the scarp may be interpretable investigate the geochemical diversity and thickness of using the rover's imaging spectrometer insofar as a section that individual strata within the drainage Valleys upon the flanks of cuts through the surface layers of Elysium Mons lava flows and Hecates Tholus without returning samples of these materials the underlying megaregolith may be visible. In addition to this to Earth. Particularly if this spectrometer were to operate in compositional mapping, which may identify volatile-rich the 5-12-J.Lm wavelength region (where a number of sediments that are important for the analysis of paleoclimates characteristic silicate emission features exist), it should be (Markun, 1988), it is also possible that geomorphic evidence possible to determine whether the inferred explosive eruptions (e.g., stream channels, moraines, or freeze/ thaw phenomena) were due to variations in magma cl1emistry or the inclusion might be found that would help resolve the mode of forn1ation of nonjuvenile volatiles within the magma (Mouginis-Mark et of the canyon system. ai., 1982), and the approximate magnitude of the eruptions After completion of the geological objectives at Station 6, it ( Wilson et at., 1982). is possible that the rover might be able to explore further the canyon system along the base of the boundary scarp and investigate the local stratigraphy of the wall rock Such a task Station 5 may be more hazardous than the earlier parts of the traverse, Upon leaving the lower flanks of ijecates Tholus, the rover so that this exploration is reserved until the end of the will traverse the boundary between the lava flows of Elysium proposed mission in case the rover is lost. Within the canyon Mons and the plains units that com~1 se part of Utopia Planitia system it is possible that in addition to Elysium Mons lava flows in the northern plains of Mars (Fig. 1 ) The boundary between and the topmost layers of the megaregolith, distal layers of ash these two surface units cannot be located to better than from Hecates Tholus also may be found as the sUlllmit of the - 10 km on the available Viking Orbiter images due to volcano is only - 120 km from the scarp. If this is the case, insufficient image resolution, but i ~ is clear that during a such ash deposits may be spectroscopically identified, traverse of - 40 km the morpholo~ of the surface changes permitting further assessments of the eruptive history of the from subdued lava flows in the southwest to smoother plains volcano to be made [specifically, placing constaints on the materials in the northeast. These mkterials in the northeast dispersal of ash from the volcano, which pertains to the height appear to overlie a unit that is now llexposed only as a series of the eruption column in plinian eruptions; Wilson et ai. of isolated massifs (Fig. 4), as well as to possess numerous ( 1982), Mouginis-Mark et ai. ( 1988)]. enigmatic sinuous ridges. These ridge~ have been hypothesized by Luccbitta et ai. ( 1986) to be comP,ressional features formed SURFACE PROPERTIES in sediments, while Parker et at. ( 987) have documented En route Topography morphologically similar features elsef here on Mars that they compare to tombolos on Earth (b":9 created by wave action While little is known about the meter-scale topography to on open bodies of water that deposit sediments at the be expected at this locality on Mars, it is likely that a shoreline). combination of information from the two Vtking Landers · Vo lcanic materials in Elysium 549
Fig. 5. The distal end of the canyon system associated with the breakup of the cap-rock along the boundary of Elsyium Planitia and Utopia Planitia marks the location of Station 6 (located at 33.97° N, 21 Ll9°W). It is expected that at this location the stratigraphy of the escarpment could be observable to the rover's imaging device, and morphological evidence may be found to help in the interpretation of volcano/ground-ice interactions such as the release of basal melt water foUowing the emplacement of the lava flows from Elysium Mons. VIking Orbiter image 86A37, image resolution 38m/pixel. Image width is equivalent to 40 km and north is to the top. 550 Proceedings of the 20th Lunar and Planetary Science Co nference
(Mutch et al., 1977; Garvin et al. 1981), Viking Orbiter ThBLE 1. Travel distances between science stations. thermal inertia measurements ( Cbrls_fensen, 1986, 1988), and Location Distance common experience with terrestri~ volcanic plains (Aubele and Crumpler, 1988; Garvin and Zither, 1988) can be used Landing Site to Station 1 (round trip) 15 km to predict the topography that a rov; r would have to traverse Landing Site to Station 2 (round trip) 13 km Landing Site to Station 3 81km during the collection of samples fo return to Earth (i.e., at Station 3 to Station 4 (upslope) 11 km the landing site and Stations 1 and 2 and during the traverse Station 4 to Station 5 (downslope) 26km from the landing site to Station 3 and beyond. Station 5 to Station 6 18km From Viking images that have a re~lution of 50 m/pixel, the Beyond Station 6 (canyon exploration) tltis morphology of the Elysium Mons lava flows in area appears Total Distance 164km to be similar to that of flows found in the Tharsis region, where small-scale ( - 50 m horizontal spaciftg) pressure ridges and flow channels have been found (Tbeilig and Greeley, 1986). Analysis of the Earth-based radar data for some of the lava flows continuing up the flank of Hecates Tholus to Station 4, and in Tharsis (Schaber, 1980) shows tha these Tharsis flows have then on to Stations 5 and 6, both gravity measurements and very high surface roughness values qems slopes in excess of active seismic experiments could help resolve the subsurfuce 3°-4° ), so it is possible that the El}'sium lava flows may be structure of the area and provide information on the original quite rough. However, because the Elysium flows are structure of Hecates Tholus volcano. apparently older than the Tharsis lava flows (Scott and Tanaka, Based on numerical modeling of martian pyroclastic flows 1986; Greeley and Guest, 1987), it is also possible that the (Wt'lson et a/., 1982; Mouginis-Mark et a/., 1988), it is surfaces of the flows along the prop,osed rover traverse may expected that such volcanic flows may have traveled about not have such a high surface rou~ess provided that the 350-400 km from the vent. Although it seems likely that weathering rates in Tharsis and Elysiwp are comparable. Hecates Tholus has experienced explosive eruptions The travel distances between each science station are ( Mouginis-Mark et al., 1982 ), geomorphic evidence only exists presented in Table 1. From inspection of Fig. 1, it is evident to support the hypothesis that plinian eruptions (producing that in order to find a "safe" lan~g site and sample the ash falls rather than pyroclastic flows) took place on this possible jokulhalup deposit at Stationj2, a total rover range of volcano. Such ash fall materials are· likely to be widely the order of at least 164 km is required. In addition, this range distributed at distances in excess of - 100 km from the vent, estimate ignores possible diversions that may be needed to and their distribution should place an upper limit on the basal navigate across the (possibly) rugged surface of the lava flows diameter of the volcano. Thus, if a buried lower flank of in this area. From the perspective of groviding an absolute age Hecates Tholus were revealed by seismic measurements from date of Lower Amazonian mateials, dbtermining the petrology the rover, it would indicate that another form of volcanism, of the lava flows and gaining an understanding the cause( s) either subplinian pyroclastic flows or effusive activity, took of very long run-out lava flows on Maif', returned samples from place. the landing site and the lava flow ~~ Station 1 would be the Observational data and theoretical models indicate that the highest priorities. The need for samp~g the jokulhalup deposit shallow martian crust is likely to contain several discontinuities is less pressing. If confirmed as a mudflow, this deposit would (Davis and Golombek, 1989). From the analysis of the provide evidence of relatively rece~t near-surface volatiles. available Vtking Orbiter images, it is possible to construct an Moving the landing site 18 km to ilie east of the proposed idealized cross-section from the lower exposed flanks of location and deleting Station 2 woul reduce the total length Hecates Tholus to the isolated mesas north of the boundary of the traverse to Station 6 to 135 F without significantly scarp (Fig. 6). The seismic and gravity data from the rover affecting the hazard level of the Iandin site. should provide measurements of the basal diameter of Hecates Tholus and the thickness of the Elysium Mons lavas in this area. In addition, spectroscopic measurements made from the rover En route Science on leaving Station 6 may also help resolve whether outliers of Key geological observations can o be made during the Elysium Mons lavas are preserved as cap-rock on the mesas drive between several of the primary science stations. Aubele north of the boundary scarp, improving our knowledge of the and Crumpler ( 1988) have proposed a model for the in situ maximum radial extent of effusive volcanism from Elysium weathering of martian lava flows, based on observations of Mons. terrestrial flows. If this model is corr 1ct, it should be possible A further science target during the drive from Station 4 to to obtain, in a relatively short p riod of time, imaging Station 5 is the ejecta blanket of the 5-km-diameter impact spectrometer data of chip fragments of the flows that could crater located at 33.6°N, 211.1 °W. IJke most fresh martian provide information on the heterog1neity of the lava flows meteorite crate~ of this size, this crater has fluidized, lobate during the traverse from Station 2 ~o Station 3. Depending ejecta deposits ( cf. Carr et al., 1977b). Several secondary upon the science instrumentatio onboard the rover, craters can also be found around this crater in addition to the important observations could also be made at the boundaries ejecta lobes, which is somewhat unusual for martian craters between the Elysium Mons lava flows, Hecates Tholus, and the of this diameter (Schultz and Singer, 1980), and suggests that northern plains. For example, from 0 km west of Station 3, the primary crater ejected both coherent blocks and more MouginJ Mark: Volcanic materials in Elysium 551
A' A
ELYSIUM MONS LAVAS LAVA-CAPPED MESAS t -3km? '
40km a Fig. 6. (a) Hypothetical cross-section extending from Station 4 to the boundary scarp beyond Station 6 (see b for location). Of particular interest is the determination (via active seismic or gravimeter experiments) of the maximum hbrizontal extent of the flanks of Hecates Tholus volcano, and the thickness of the lava flows from Elysium Mons. In this sketch the depth of excavation of the 5-km-diameter crater is schematic, but the determination of the depth of the breccia lens could be of value in the analysis of martian impact craters with fluidized ejecta deposits (cf Mouginis-Mark, 1987). (b) Location of the cross-section (shown by dashed line) illustrated in (a). The positions of Stations 3-6 are also shown. Mosaic ofVtking Orbiter images 86A35-40, resolutibn 38m/pixel. Image width is equivalent to 68 km and north is to the top. 552 Proceedings of the 20th Lunar and Planetary Science Conferen ce
fluidized materials. The opportunity t? study the morphology Some information on physical properties of the martian of the deposits around this crater may help resolve some of surface was obtained by the Viking Landers (e.g., Sborthill et the issues relating to the role of target volatiles (Mouginis at., 1976; Moore et at., 1982), but remote-sensing data indicate Mark, 1987) or atmospheric effect (Schultz and Gault, that the Viking Lander sites are distinctive from most of the 1979) in the formation and emplacement of martian ejecta rest of Mars Uakosky and Christensen, 1986). It is inferred blankets. here that it is likely that the undifferentiated lava flows that make up the northern Elysium Planitia landing site will be able CONCLUSIONS AND RECOMMENDATIONS FOR A to support the lander and permit the rover to reach the other FUTIJRE MARS ROVER two return sample sites (Stations 1 and 2). What is not so obvious is the bearing strength of the flanks of Hecates Tholus This study was motivated by the need to understand further (Station 4), which on morphologic grounds could comprise the local diversity of volcanic material~ on the martian surface relatively unconsolidated ash deposits or nonwelded pyroclas in order to resolve differences in the styles of volcanism and tic flows (Mouginis-Mark et at., 1982, 1988). In order to the role of volcano/ground ice interactions. In many instances answer these and other pert.inent engineering and geologic the geologic objectives within Elysium Planitia would require issues for a variety of candidate landing sites on Mars, it a surface rover to adequately interpret the emplacement therefore appears appropriate that new efforts be made to process and stratigraphy of units identified. The landing site provide such information (via photogeologic and remote described here does not, however, peljlllit samples of very old sensing means) at this early stage of planning for a rover (or very young) martian rocks to be collected, based upon our mission. current knowledge of martian stratigraphy and chronology (Scott and Tanaka, 1986; Greeley and Guest, 1987). Acknowledgments. This research was supported by a grant from Nevertheless, returning samples that Will permit an absolute the Mars Geologic Mappers Program, NASA Grant No. NAGW-1345. age for the Unit Ael 1 lava flows surrounding Elysium Mons will I thank J. E Bell for his comments on an earlier version of this be of major value in defining the martian crater chronology manuscript, K Tanaka and R De Hon for formal reviews, and J. curve. Thus, the northern Elysium site described here should Zimbelman for editorial assistance. This is Hawaii Institute of be considered as an example of an area where fundamental Geophysics Contribution number 2188. geologic and volcanologic research c~uld be conducted by a rover with a 200-km range and smap~e return capability, and it is emphasized that this site should not be the only one considered during planning for a Mars rover/sample return REFERENCES mission. Albee A. L. ( 1988) The sample site on Mars (abstract). In Workshop What is clear from this study, however, is the need for the on Mars Sample Return Science (M. J. Drake et al., eds.), pp. 28- rover to reach specific exposures or surface materials, even 29. LPI Tech. Rpt. 88-07, Lunar and Planetary Institute, Houston. though such sites may require traverses over relatively rugged Allen C. C. ( 1979) Volcano-ice interactions on Mars. J Geophys. Res., lava flows and an ascent of several undred meters. As the 84, 8048-8059. trafficability potential of the rover increases, a greater diversity Aubele J. 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